1. Field of the Invention
The present invention relates to a furnace structure. More particularly, the present invention relates to a heat insulation pedestal and a vertical type furnace tube.
2. Description of the Related Art
In the fabrication of semiconductor devices, many steps are often involved. In some of these steps, prolonged high-temperature treatment is frequently required in process such as thermal diffusion. At present, most thermal diffusion processes are commonly carried out using vertical type furnace tubes because a vertical furnace tube typically occupies very little space.
FIG. 1 is a cut away view showing the structure of a conventional vertical type furnace tube. As shown in FIG. 1, the vertical furnace tube 100 mainly comprises an outer furnace tube 102, an inner furnace tube 104, a wafer boat 106, a heat insulation pedestal 108 and wafer boat holder 110. Before carrying out the processing, wafers 112 are placed inside the wafer boat 106 and then the wafer boat 106 is raised from the bottom section of the inner tube 104 into the interior of the inner tube 104. The pedestal 108 is located inside the inner tube 104 below the wafer boat 106. The wafer boat holder 110 is set up between the wafer boat 106 and the pedestal 108.
After performing a conventional high-temperature thermal treatment such as a well diffusion process inside a conventional vertical furnace tube, some of the wafers are broken as a result of abnormal transport. On close examination, the principal reason is found to be the deformation of the heat insulation pedestal. The cause of pedestal deformation is explained in more detail in the following with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B are the perspective view and the side view of section II of the pedestal in FIG. 1. As shown in FIGS. 2A and 2B, the heat insulation pedestal 108 comprises a top support 202, a plurality of heat insulation plates 204 and a number of connection sections 206. The top support 202 is used for supporting the wafer boat 106 and the wafer boat holder 110 (see FIG. 1). The heat insulation plates 204 are set up under the top support 202. The connection sections 206 connect various heat insulation plates 204 to the top support 202. The heat insulation pedestal 108 deforms mainly because the wafer boat 106, the wafer boat holder 110 and the wafers 112 (see FIG. 1) weigh down on the top support 202 during a lengthy high temperature processing operation. The deformation of the top support 202 often leads to a tilting of the wafer boat 106 and hence increasing the likelihood of wafer damage 112 during transportation.
To prevent damage to the wafer products, the heat insulation pedestal of the vertical type furnace tube in a production line must be replaced at fixed interval. However, replacing the pedestal not only costs a lot of money but also wastes a lot of processing time. Ultimately, productivity of the thermal processing operation is reduced.